Even when a particular biomass material is suitable for straightforward burning as described in Chapter 7, this will not necessarily be the most efficient way of recovering the embodied energy of the biomass. Materials with naturally high moisture content in particular may well contain significant amounts of hydrocarbons and other organic materials; but drying them to a state in which they could be fed directly into a boiler is rarely either practical or economic. However, alternative methodologies are available, some of which have been around a very long time. These can transform basic biomass into more convenient fuels with a greater energy density that can burn cleaner and at a significantly higher temperature.
For example, as a fuel, wood from the forest leaves a lot to be desired; timber from most species of tree needs to air-dry for a considerable period before burning, otherwise excessive smoke is produced. The moisture in the wood limits the maximum flame temperature that can be attained. Tarry residues from oils and resins in the wood clog up chimneys and flues. In the search for a better fuel, sometime in the distant past people learned how to convert tree wood into charcoal, which burned hotter and gave off much less smoke. The process of charcoal burning, crude as it originally was, was the first systematic use of what we now call pyrolisation.
I n essence, pyrolisation occurs when organic material is heated in the absence of air. Gases and liquids are driven off to leave a purified char behind. The production of charcoal originally involved the slow combustion of part of the wood in a large pile sealed with clay or turf. Some air is needed for the process to work, but air supply is carefully restricted to the absolute minimum. Charcoal is the only end product; however, if wood is heated in a sealed container, it will yield charcoal, tar, wood spirit (methanol), turpentine, pyroligneous acid - also known as wood vinegar - and wood gas, an inflammable gas containing hydrogen and carbon monoxide, which can be used as fuel for the heating process. This process is also known as destructive distillation, and played a major role in industry for many centuries. The tar was essential to the shipping industry as a preservative coating, turpentine was a valuable solvent, wood vinegar could be refined into acetic acid and the production of steel and gunpowder depended on large quantities of charcoal. However, increasing demand caused massive deforestation, especially in Central Europe, and increasing scarcity stimulated a swing to coke and coal tar - also produced by the pyrolisation process.
Most biomass materials can be pyrolised in the absence of oxygen. All will yield similar combinations of char, liquids and gas - the gas is usually dubbed syngas, and is used as the basic fuel in most pyrolisation plants. The liquid is bio-oil, which can be used directly as a heating fuel, or can be processed into raw materials for pharmaceuticals or petrochemicals. Chars can also be burnt, or used as a soil conditioner and fertiliser, locking up significant amounts of carbon in the process.
Fast pyrolysis technology has been developed in the Netherlands by the Biomass Technology Group. At the heart of the process is the rotating cone reactor. Fine particles of biomass and hot sand are introduced at the bottom of the cone, and pyrolysis occurs as the reactor rotates at 300 rpm, spinning the biomass upwards. Bio-oil is the main product: the heat required comes from the separate combustion of the char produced.
A more energy efficient alternative is torrefaction . This is effectively pyrolisation at a lower temperature than that normally used for charcoal making, and the process can cope with a wider variety of feedstock types and moisture contents. The solid end product, sometimes known as biocoal, is more predictable, less prone to dusting and is effectively resistant to reabsorption of moisture when stored. It can be burned directly, or used to produce pellet fuels and barbeque briquettes, and has shown good results as a gasifier fuel (see below). An integrated process in which airless drying with superheated steam prepares the biomass for the high temperature stage is also available. The steam comes from the biomass itself, the heating from the gas driven off in the torrefaction process. Moisture content of the 'smokeless' solid end product is 3%, original mass is reduced by around 30% but 90% of the initial calorific value is retained.
In the nineteenth century it was discovered that by admitting limited amounts of air and/ or steam to the pyrolisation chamber the volume and energy content of the gas produced
was significantly increased. A three-stage process is involved. Straightforward pyrolisation occurs first, releasing volatile liquids and gases and leaving char behind. Then combustion begins, as the volatile products (and some of the char) react with the oxygen in the air, raising the temperature and producing carbon monoxide and carbon dioxide. Finally, the char reacts with the carbon dioxide and steam - which can come from the moisture content of the biomass - to yield more carbon monoxide and hydrogen. The end product -usually known as producer gas - will have lesser or greater percentages of contaminants such as tars, alkalis, sulphur, ammonia, chlorine, and particulates, and may well need to be cleansed before further use.
At the beginning of the twentieth century wood or producer gas was a widely used alternative to town gas, which came from the gasification of coal. Petroleum shortages during both World Wars boosted the demand for gas as an alternative if less effective fuel for motor vehicles. Un-pressurised gaseous fuels have a much lower energy density than liquid fuels, so the near 1 million vehicles around the world that were converted to run on gas were immediately recognisable, thanks to the large inflated gas bags on their roofs. Cheaper fossil fuels in the 1950s and 1960s largely killed off interest in gasification, only for it to be revived by the 1973 oil crisis.
The terms 'producer gas' and 'wood gas' are often interchangeable, reflecting the reality that most non-coal gasification traditionally used either wood or charcoal as the raw material. Charcoal was the simplest option, as its gasification yielded the minimum of ash,
tars and other unwanted contaminants, but this was a low efficiency process. Charcoal production wasted at least 50% of the energy in the wood. These days, most modern gasification plants now use some form of wood, either recycled, wastes from forestry or arboriculture, or specially grown (see Chapter 7). An initial drying stage is usually included in the gasification process, unless torrefied wood or biocoal are used.
Gasification of wood is reckoned to yield around 2.5 m3 of producer gas for every 1 kg of wood consumed - this gas will have nearly 70% of the calorific value of the original wood. Other types of biomass can be gasified and performance is similar, but experience is still limited and results are mixed. There is often a useful by-product in the form of a char that can be processed into briquettes that can replace firewood for cooking - or be used as a soil improver.
A number of manufacturers worldwide now offer a range of gasification plants. The most popular and longest established variant is the updraft gasifier, also known as the counter current fixed bed and, confusingly, the counter current moving bed gasifier. Biomass enters the reactor at the top, steam, oxygen and/or air is blown in at the bottom below a grate. The biomass falls down against the upward flow of the gases until it reaches the grate at the bottom. During this transit the biomass is progressively dried, pyrolised, chemically reduced and finally combusted.
For the process to work the biomass has to have significant mechanical strength and be non-caking, so that it can form a permeable bed of red hot char through which the incoming gases can flow freely. Throughput is low, thermal efficiency is high - but so is the tar content of the gas, as gas exit temperature is relatively low and condensation of volatiles is inevitable. Slag production is low. Most updraft gasifiers operate at atmospheric pressure. One interesting variation inserts a gas combustor above the gasifier, which burns the hot producer gas as it leaves. The flue gas is then piped directly to the heater head of a Stirling engine, best described as an external combustion piston engine (see Chapter 14). This in turn drives an electrical generator.
A more efficient alternative, especially in smaller sizes, is the downdraft, or co-current, fixed bed (moving bed) gasifier. As the name suggests, the air flows in the same direction as the movement of the biomass, which still enters from the top. Producer gas is drawn off from the bottom. Downdraft gasifiers tend to be significantly taller than the updraft alternative, and are unable to cope with very variable biomass or small particle sizes. Against this, the gas will leave at a much higher temperature, reducing the tar content, and thermal efficiency is on a par with the updraft design.
Fluidised bed gasifiers are generally much more tolerant to variations in the biomass supply, at the cost of greater complexity. The biomass is suspended in high-pressure air blasted up through a sand bed. Mixing is vigorous, with all stages of gas production taking place simultaneously. Tar content of the gas is intermediate between updraft and downdraft gasifiers, and the process is somewhat more difficult to control.
Gasifiers using air as the oxidant, or gasification agent, and operating at atmospheric pressure yield gas with a significant nitrogen content, which lowers its calorific value. Typically, producer gases from this type of installation will have a calorific value in the range 2.5-8.0 MJ/Nm3. Much higher calorific values - up to 20MJ/Nm3 - can be obtained by using oxygen instead of air, and/or operating the gasifier at high pressure, up to 16 bar. This obviously increases the complexity of the process by several orders of magnitude. Gasification equipment is still comparatively expensive, and small-scale gasifiers are unlikely to be economic unless the biomass supply is effectively free.
Raw producer gas may be burnt in furnaces and boilers without further treatment, and this is perhaps the best route where the biomass/gasifier combination used yields gas with a high tar content. The heat produced can be utilised for a wide range of purposes.
ENERGY FROM WASTE PLANT
1 Fuel bunker
2 Fuel crane
3 Screw conveyer
4 Primary chamber (Gasification)
5 Secondary chamber (High temperature oxidation)
6 Heat recovery steam generator (HRSG)
7 Lime and carbon silo
8 Bag house filter
9 Filter residue sile
10 Flue gas fan
13 Steam turbine
14 Air cooled condenser
Normally the raw gas will be cooled before storage/use, to increase its energy density. Electricity generation using producer gas is said to be around 20% more efficient than generation based on the direct combustion of biomass. The concept of using producer gas to fuel a cogeneration (combined heat and power) plant has been attracting a lot of attention, as this promises overall thermal efficiencies of more than 80%, as compared to direct combustion's 60%. On the smaller-scale, this usually involves converting the chemical energy of the producer gas into electrical energy by using it as fuel in some form of internal combustion engine, which then drives an electricity generator (see Chapter 14). The problem is that most internal combustion engines demand a very clean gas to function effectively, so that tars in particular must be removed before the gas is acceptable.
This has proved to be the Achilles' heel of many pioneering gasification installations used for cogeneration. Moisture content of the gas is fairly easy to control; if the preliminary drying stage takes the initial moisture content of the biomass to below 20%, the moisture content of the gas leaving the system is usually acceptable. Dust is normally removed through a combination of filters. The tars or condensates are a much bigger challenge, and work continues on the development of a reliable, cost-effective and energy efficient tar removal system.
In the USA there has been significant progress in using plasma torches to raise the temperature within a classic downdraft fixed (moving) bed gasifier to 1,250°C; this 'cracks' the tars and other volatiles, reducing them to hydrogen and carbon monoxide. Any inorganic material in the biomass is vitrified; all the carbon is converted to gas, leaving no char. Other systems use a combination of high temperatures and oxygen injection to achieve the same effect. Although oxygen minimises the nitrogen content of the gas - and hence enhances its calorific value - the technology is more complex and expensive and still not fully developed.
An alternative approach is adopted in the Batelle/FERCO process. No oxygen is used. Instead there are two, physically separate, reactors. Dried biomass passes first into an updraft gasifier, then the residual char passes into a second combustor, where it is burned to provide the heat for the gasification process. Heat transfer between the two reactors is accomplished by circulating sand, and throughput is said to be much higher than in other systems.
Conventional spark ignition internal combustion piston engines, i.e. the type of engine that is mass produced, cheap, reliable and rugged, can easily be converted to run on clean producer gas. Power output drops significantly - by up to 50% - thanks to the lower calorific value of the producer gas compared to petroleum products. Producer gas can also co-fuel the even more rugged and reliable compression ignition internal combustion piston engine - better known as a Diesel engine. To ensure effective ignition, only 80% or less of the diesel fuel can be replaced by producer gas, but this, and the more appropriate lower speed of diesel engines, means that overall power drops only by about 15 to 30%. Carbon monoxide emissions can be a problem, however.
Despite their lower efficiencies, piston engines are popular because of their low capital cost, ease of maintenance and availability of spares. On larger installations gas turbines are the norm. Potential efficiencies are significantly higher, but so are capital and maintenance costs. Microturbines operating at up to 100,0000 rpm and producing between 25 and
500 kW are a promising development. And for combined heat and power - cogeneration -and similar installations, the optimum choice may well be a modern version of the venerable Stirling engine (see Chapter 14).
The anaerobic digestion of biomass produces a much cleaner gas. Usually known as biogas, this is a mixture primarily of methane and carbon dioxide, with nitrogen content below 10%. There are no tars or other volatiles; however, some types of biomass will yield biogas with significant siloxane content. Siloxanes are organosilicon compounds (and when burnt release silicon), which can form abrasive deposits within internal combustion engines fuelled with siloxane-contaminated biogas. Even Stirling engines can be affected, as the deposits build up in and around the heat exchanger and are very difficult to remove.
Carbonic acids and alcohols
Hydrogen Carbon dioxide Ammonia
Carbonic acids and alcohols
Hydrogen Carbon dioxide Ammonia
Hydrogen Acetic acid Carbon dioxide
Hydrogen Acetic acid Carbon dioxide
Methane Carbon dioxide
Anaerobic digestion schematic
Siloxanes are usually found only in sewage. Anaerobic digestion has been used for many years to treat sewage sludge and animal waste, especially in the form of slurries. In many cases it yields not only biogas but also valuable by-products such as soil improvers and fertilisers - although the levels of heavy metals and pesticides have to be monitored closely. It is a basically simple process that can work on the smallest and the largest scales and needs no external power supply. Biomass with moisture contents in excess of 75% can be effectively processed. In recent years other types of waste have been tried or at least experimented with, mainly materials, such as the waste from food processing and beer and spirit production, which have moisture contents too high to make them economic for other forms of use. Energy crops such as ryegrass are also used, either on their own or as a boost to low carbon content wastes. In practice, there are two distinct types: wastes and energy crops which have negligible contents of such undesirables as heavy metals, and unsorted wastes, where the levels of contaminants are such that the residues left after digestion have no practical value and will need further treatment.
Many different species of bacteria are needed for anaerobic digestion. They gradually break down the complex organic molecules in the biomass, first into simple sugars and amino and fatty acids, then into the simpler molecules of ammonia, carbon dioxide and hydrogen sulphide. Further digestion yields hydrogen and acetic acid. Finally, methano-genesis occurs, methane and water and carbon dioxide is produced. All of these reactions take place in a sealed container in the absence of air. In the most common type of digester, only limited additional heat is needed, and mesophilic bacteria perform the digestion. The optimum operating temperature is around 40°C, but digestion will occur over the range 20-45°C. Simple batch type digesters are sealed for up to 30 days. Biogas production peaks in the middle of this period, gradually declining as the reactive materials are exhausted. Up to 60% of the biomass will be converted to biogas.
Most modern anaerobic digesters use a continuous process with regular mixing of the biomass, which results in a much more consistent production of biogas. Even so, the biomass will spend up to 18 days actually inside the digester. Mesophilic digestion will not kill all of the pathogens that might be present in the biomass; so there is often a preliminary pasteurisation stage where the biomass is heated to at least 70°C for an hour or so. Biogas burners normally supply the heat for pasteurisation and digestion. Apart from the biogas, the process also yields a slurry known as digestate, which can be separated into a fibrous, compost-like material with a significant calorific value and a nutrient-rich liquid, both of which are potentially valuable products in their own right - provided the biomass used was relatively clean. Otherwise the solid residues will have to go to landfill and the liquid will need further treatment.
Thermophilic bacteria operating at 50°C or more can digest biomass much quicker, but the process is less stable and requires more added energy. In hotter climates very small batch mesophilic digesters fed with animal and human wastes have proved to be very successful without any added heat at all. There are believed to be more than 2 million such digesters in India alone, attached to individual households and supplying low calorific value gas for lighting and cooking.
Biogas can also be used directly for space heating and cooling, or as fuel for a cogenera-tion plant (see Chapter 14). Produced by modern digesters, its calorific value is around 20MJ/m3. Anaerobic digestion will not be an attractive option for many projects despite the quality of the gas it yields. The economics of very small plants are unlikely to be attractive. In urban locations particularly, all biomass technologies may run into local objections based on the need for frequent deliveries and, in the case of anaerobic digestion, the inevitable concern about unwelcome odours.
However, for the right projects (such as industrial parks) where cheap clean biomass is available locally and space for storage and processing can be found, the conversion of biomass into a combustible gas is one of the greenest options currently available. It is an option that will become increasingly practical as technology improves.
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